This invention relates to vibration damping, to compositions having particular use for vibration damping, and to constrained layer damping devices.
It has long been known that certain viscoelastic compositions have utility in damping the vibration of substrates to which they are applied. It has also been known that viscoelastic compositions are particularly effective in so-called "constrained layer" damping structures, where a thin layer of viscoelastic composition is bonded to the vibrating substrate, with a relatively inextensible "constraining layer" bonded to the exposed surface of the composition; see, e.g., U.S. Pat. Nos. 3,537,717, 3,640,833, 3,640,836, 3,640,839, 3,674,624, 3,674,625, 3,833,404, 3,847,726, 4,223,073, 4,408,238, 4,447,493, 4,681,816, and 4,760,478.
Copolymers of non-tertiary alkyl acrylates and polar monomers (e.g., iso-octyl acrylate:acrylic acid copolymers) are known to possess viscoelastic properties that are suitable for damping. It is also known that as the amount of polar monomer increases, the peak damping temperature (i.e., the highest temperature at which the copolymer damps effectively) increases. Such copolymers are thus useful in damping the vibration of structures associated with internal combustion engines, where the working temperature approximates that of boiling water, or 100.degree. C.; indeed, oil may reach 150.degree. C. Unfortunately, however, as the amount of polar monomer increases above 20%, the copolymer becomes increasingly brittle and susceptible to shock at room temperature or below. Thus, for example, when a constrained layer damping structure based on a high acid iso-octyl acrylate:acrylic acid copolymer is applied to the oil pan or tappet valve cover of an automobile, any shock encountered during the warm-up period is likely to cause the structure to loosen or even fall off. Regardless of how effective such a structure might have been at elevated temperatures, it will, of course, be considered unsatisfactory.